1. Field of the Invention
The invention relates to a motor control circuit, and more specifically to a motor control circuit with reduced noise and control method thereof.
2. Brief Discussion of the Related Art
Current motor controlling techniques had been a highly developmental technology. In the related art, circuits exist which not only control the start-up of motors but also change and limit the motor speed in addition to other functions.
For example, FIG. 1 shows a conventional motor control circuit. As depicted, the motor control circuit 1 comprises a driving circuit 10, a Hall element 12, a coil switching circuit 14 and a pulse width modulation generator 16. Hall element 12 detects signals generated by the motor (not shown in the figure), such as the phase switching signal generated during the rotation of the motor, and generates a phase signal wherein the phase thereof is consistent with the rotation phase of the motor. Driving circuit 10 is coupled to Hall element 12 and receives the phase signal from hall element 12 to generate a motor speed control signal which usually comprises a plurality of continuous and periodic square pulses. FIG. 2 shows a pulse diagram of motor speed control signal 21, wherein the y-axis A denotes the amplitude of signal 21 or voltage in this example and x-axis t denotes the time. Moreover, driving circuit 10 controls the motor to stop, limits the motor speed and even controls other functions. Driving circuit 10 is generally made up of an application specific integrated circuit (ASIC) which is familiar to get for the person skilled in the art. Coil switching circuit 14 usually comprises two MOS switches. When receiving the motor speed control signal from driving circuit 10, coil switching circuit 14 outputs the motor speed control signal to the coil 20 in the motor accordingly and thereby switches the current directions in two neighboring phases of the coil 20, maintaining rotation of the motor.
Further, a pulse width modulation (PWM) generator 16 is provided to modulate the motor speed control signal into a plurality of small periodic square pulses as shown in FIG. 3. According to the equivalence principle, the amplitude of the equivalent motor speed control signal is smaller than that of the original motor speed control signal as shown in FIG. 4 and thus the motor speed is decreased when the equivalent motor speed control signal is input to the coil 20 in the motor. The method of decreasing motor speed relates to modulating the square pulses in pulse width modulation but as this is a related art method, it is not described in further detail here.
When the phase of coil 20 in the motor is changed, that is, the current direction is changed as shown by point P in FIG. 2, a very loud noise typically occurs with such motor speed control signal, particularly when used as a fan motor. Thus, the usage of this type of fan motor is therefore limited.